1. Field of the Invention
The invention relates to power converters, and more particularly to synchronous power converter of power converters.
2. Description of the Related Art
An offline power converter includes a power transformer to provide isolation from an AC line input to the output of the power converter for safety. In recent development, using soft switching topologies in the primary side of the transformer and applying the synchronous rectifier in the secondary side of the transformer are to reach a higher efficiency for power conversion. Among them, the full-bridge quasi-resonant ZVS techniques are described in “Constant frequency resonant power converter with zero voltage switching” by Christopher, P. Henze, et al, U.S. Pat. No. 4,855,888; “Soft-switching PWM converters” by Guichao C. Hua and Fred C. Lee, U.S. Pat. No. 5,442,540; “Zero switching power converter operable as asymmetrical full-bridge converter” by Yang, et al, U.S. Pat. No. 6,744,649. The drawback of foregoing power converters is low efficiency at light load. An insufficient circular power causes hard switching and results low efficiency. The purpose of using the synchronous rectifier at the secondary side is to reduce the power loss of rectifiers. The description of the synchronous rectifying can be found in a prior art of “Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter” by Yang, U.S. Pat. No. 7,173,835. The disadvantage of this skill is extra devices such as saturable inductors and current-sense resistor cause additional power consumption. The object of the invention is to integrate the synchronous rectifying circuit with the regulation circuit to achieve higher efficiency. No further switching stage is needed. The primary-side switching circuit, the secondary-side synchronous rectifier and the regulation circuit achieve high efficiency power conversion from no load to full load.